Coated vertically aligned carbon nanotubes on nickel foam

ABSTRACT

Vertically aligned carbon nanotubes (VACNTs) (e.g., multi-walled VACNTs and methods of synthesizing the same are provided. VACNTs can be synthesized on nickel foam (Ni—F), for example by using a plasma-enhanced chemical vapor deposition (PECVD) technique. A wet chemical method can then be used to coat on the VACNTs a layer of nanoparticles, such as tin oxide (SnO 2 ) nanoparticles.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application of U.S.application Ser. No. 17/471,313, filed Sep. 10, 2021, the disclosure ofwhich is hereby incorporated by reference in its entirety, including allfigures, tables, and drawings.

GOVERNMENT SUPPORT

This invention was made with government support under DMR-1506640awarded by National Science Foundation (NSF). The government has certainrights in the invention.

BACKGROUND

Lithium (Li)-ion batteries (LIBs) have become an essential part ofeveryday life due to their increasing applications, extending fromportable electronics to electric vehicles. LIBs are one of the energystorage and power devices based on electrochemical energy storage andconversion mechanisms. Owing to the excellent properties of LIBs, suchas superior energy density, a broader operating temperature range, a lowself-discharge rate, and devoid of memory effect, they are superioramong other battery technologies. However, the ever-increasing demandfor LIBs capable of delivering high energy and power densities withminimal volumetric constraints and safety issues requires better LIBcomponents with enhanced electrochemical properties.

BRIEF SUMMARY

Embodiments of the subject invention provide novel and advantageousvertically aligned carbon nanotubes (VACNTs) (e.g., multi-walled VACNTs(MW-VACNTs) and methods of synthesizing the same. VACNTs can besynthesized on nickel foam (Ni—F), for example by using aplasma-enhanced chemical vapor deposition (PECVD) technique. A wetchemical method can then be used to coat a layer on the VACNTs, forexample a layer of nanoparticles such as tin oxide (SnO₂) nanoparticles.SnO₂-coated VACNTs on Ni—F have excellent lithium (Li)-ion storagecapacity and can be used as a material for an anode in high-performanceLi-ion batteries (LIB s).

In an embodiment, a method of synthesizing VACNTs can comprise:providing a metal substrate (e.g., a Ni—F substrate (e.g., athree-dimensional (3D) Ni—F substrate)) in a reaction chamber; loweringa pressure (e.g., to below 1 Torr, such as 0.01 Torr) of the reactionchamber (e.g., via pumping the reaction chamber) and providing a carbonprecursor gas (e.g., acetylene) to the chamber to perform a PECVDprocess to synthesize the VACNTs on the substrate; and performing a wetchemical process on the VACNTs connected to the substrate to coat theVACNTs with nanoparticles such that the VACNTs have a coating of thenanoparticles on respective outer walls thereof. The PECVD process canfurther comprise providing the carbon precursor gas in the presence ofammonia and/or performing the PECVD process at a temperature of at least400° C. (e.g., 600° C. or about 600° C.). DC plasma (e.g., of 70 Watts(W) or about 70 W) can be applied at a given pressure (e.g., 7 Torr orabout 7 Torr) while the carbon precursor gas is provided. The VACNTs canbe MW-VACNTs. The method can further comprise, after performing thePECVD process, performing an acid treatment on the VACNTs connected tothe substrate to form oxygen-containing functional groups on therespective outer walls of the VACNTs. The acid treatment can compriseproviding a first acid (e.g., nitric acid (HNO₃)) to the VACNTs for afirst predetermined amount of time (e.g., at least 10 minutes, such as15 minutes or about 15 minutes). The oxygen-containing functional groupscan comprise carboxyl groups and/or hydroxyl groups. The nanoparticlescan be SnO₂ nanoparticles. The wet chemical process can compriseproviding tin (II) chloride dihydrate (SnCl₂.2H₂O) and oxygen (O₂) tothe VACNTs. The wet chemical process can comprise: forming a precursorsolution comprising a second acid (e.g., hydrochloric acid (HCl)) andSnCl₂.2H₂O; and submerging the VACNTs on the substrate in the precursorsolution while providing O₂ for a second predetermined amount (e.g., atleast 1 hour, such as 9 hours or about 9 hours) of time such that theSnO₂ nanoparticles coat the respective outer walls of the VACNTs. TheVACNTs can be binder-free (i.e., contain no binder) and/or free ofconductive additive materials (i.e., contain no conductive additivematerials).

In another embodiment, a compound can comprise: a metal substrate (e.g.,a Ni—F substrate (e.g., a 3D Ni—F substrate)); VACNTs disposed on andconnected to the substrate; and a coating of the nanoparticles onrespective outer walls of the VACNTs. The nanoparticles can be, forexample, SnO₂ nanoparticles. The VACNTs can be MW-VACNTs. The VACNTs canbe binder-free (i.e., contain no binder) and/or free of conductiveadditive materials (i.e., contain no conductive additive materials). TheVACNTs can comprise oxygen-containing functional groups (e.g., carboxylgroups and/or hydroxyl groups) on the respective outer walls thereof.

In another embodiment, an LIB can comprise: a cathode (e.g., a cathodecomprising lithium); an anode comprising a compound as described herein(comprising VACNTs on a metal substrate (e.g., a Ni—F substrate)); andan electrolyte disposed between the cathode and the anode.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic view of synthesis of vertically aligned carbonnanotubes (VACNTs) on nickel foam (Ni—F), coating with tin oxide (SnO₂),and fabrication of a CR2032 lithium (Li)-ion coin cell, according toembodiments of the subject invention.

FIGS. 2(a)-2(d) show atomic force microscope (AFM) views of Ni—F (singlestrip) before and after a heat treatment at 600° C. under an ammonia(NH₃) environment for 6 minutes. FIG. 2(a) shows the surface of the Ni—Fbefore the heat treatment, and FIG. 2(b) shows the surface of the Ni—Fafter the heat treatment. The scale bar in each of FIGS. 2(a) and 2(b)is 2 micrometers (μm). FIG. 2(c) is a three-dimensional (3D) view ofFIG. 2(a), and FIG. 2(d) is a 3D view of FIG. 2(b).

FIG. 3(a) is a scanning electron microscope (SEM) image of VACNTs grownon Ni—F. The scale bar is 100 μm. The inset is a magnified view of theboxed area. The scale bar of the inset is 10 μm.

FIG. 3(b) is a low-magnification transmission electron microscope (TEM)image of a VACNT. The scale bar is 200 nanometers (nm).

FIG. 3(c) is a high-magnification TEM image of a VACNT. The scale bar is20 nm. The inset shows a high-resolution TEM image showing the latticefringes in the VACNT wall. The scale bar of the inset is 5 nm.

FIG. 3(d) is a TEM image at the interface between the graphitic layersof a VACNT and a catalyst particle. The scale bar is 5 nm. Theupper-right inset is the selected area diffraction (SAD) pattern of thecatalyst particle trapped at the tip of the VACNT. The scale bar in thisinset is 5 nm⁻¹. The lower-left inset is the energy-dispersive X-rayspectroscopy (EDS) spectrum of the catalyst particle trapped at the tipof the VACNT.

FIG. 3(e) is an SEM image of SnO₂-coated VACNTs (SnO₂-VACNTs) grown onNi—F. The scale bar is 100 μm. The inset is a magnified view of theboxed area. The scale bar of the inset is 10 μm.

FIG. 3(f) is a low-magnification TEM image of an SnO₂-VACNT composite.The scale bar is 200 nm.

FIG. 3(g) is a high-magnification TEM image of an SnO₂-VACNT composite.The scale bar is 20 nm.

FIG. 3(h) is a TEM image at the interface between the graphitic layersof a VACNT and the SnO₂ nanoparticles. The scale bar is 5 nm. Theupper-right inset is the SAD pattern of the SnO₂ nanoparticle. The scalebar in this inset is 5 nm⁻¹. The lower-left inset is the EDS spectrum ofthe SnO₂ nanoparticle.

FIG. 4(a) shows the thermogravimetric analysis (TGA) spectra forpristine VACNTs and SnO₂-VACNTs. The curve with the higher weightpercentage value at a temperature of 800° C. is for the SnO₂-VACNTs.

FIG. 4(b) shows the Raman spectra for pristine VACNTs and SnO₂-VACNTs.The curve on top is for the SnO₂-VACNTs.

FIG. 4(c) shows the X-ray diffraction (XRD) spectra for pristine VACNTsand SnO₂-VACNTs. The curve on top is for the SnO₂-VACNTs.

FIG. 4(d) shows the Fourier transform infrared spectroscopy (FTIR)spectra for nitric acid (HNO₃)-treated VACNTs and SnO₂-VACNTs. The curveon top is for the SnO₂-VACNTs.

FIG. 5(a) shows a plot of current (in milliamps (mA)) versus voltage (inVolts (V), versus Li⁺/Li) for pristine VACNTs as an anode material in aLi-ion coin cell, with a scan rate of 0.2 millivolts per second (mV/s)measured between 0.01 V and 3 V.

FIG. 5(b) shows a plot of current (in mA) versus voltage (in V, versusLi⁺/Li) for SnO₂-VACNTs as an anode material in a Li-ion coin cell, witha scan rate of 0.2 mV/s measured between 0.01 V and 3 V.

FIG. 5(c) shows a plot of voltage (in V, versus Li⁺/Li) versus capacity(in mA-hours per gram (mAh/g)), showing the charge/discharge profile forpristine VACNTs as an anode material in a Li-ion coin cell, with acurrent rate of 0.1 Amps per gram (A/g) within a voltage range of 0.01 Vand 3 V.

FIG. 5(d) shows a plot of voltage (in V, versus Li⁺/Li) versus capacity(in mAh/g), showing the charge/discharge profile for SnO₂-VACNTs as ananode material in a Li-ion coin cell, with a current rate of 0.1 A/gwithin a voltage range of 0.01 V and 3 V.

FIG. 6(a) shows a plot of capacity (in mAh/g) versus cycle number,showing cycle stability for pristine VACNTs.

FIG. 6(b) shows a plot of capacity (in mAh/g) versus cycle number,showing cycle stability for SnO₂-VACNTs.

FIG. 6(c) shows a plot of capacity (in mAh/g) versus cycle number,showing rate performance for pristine VACNTs.

FIG. 6(d) shows a plot of capacity (in mAh/g) versus cycle number,showing rate performance for SnO₂-VACNTs.

FIG. 6(e) shows a plot of capacity (in mAh/g) versus cycle number,showing cycling performance for SnO₂-VACNTs at a high current density of1 A/g and Coulombic efficiency.

FIG. 7(a) shows a plot of −Z″ (in Ohms (Ω)) versus Z′ (in Ω), showingthe impedance spectra for VACNTs before and after the discharge-chargecyclic process. The curve with the higher value of −Z″ at Z′=200Ω is forthe before cycles.

FIG. 7(b) shows a plot of −Z″ (in Ω) versus Z′ (in Ω), showing theimpedance spectra for SnO₂-VACNTs before and after the discharge-chargecyclic process. The curve with the higher value of −Z″ at Z′=200Ω is forthe before cycles.

FIG. 8 shows an equivalent Randle's circuit model used for fittingexperimental impedance data.

FIG. 9 shows a table of impedance parameters of VACNTs and SnO₂-VACNTsas anode materials calculated from an equivalent Randle's circuit.

FIG. 10 shows a schematic view of a plasma-enhanced chemical vapordeposition (PECVD) system that can be used to synthesize VACNTs,according to an embodiment of the subject invention. The VACNTs can beSnO₂-coated VACNTs and can be synthesized on Ni—F (e.g., 3D Ni—F).

DETAILED DESCRIPTION

Embodiments of the subject invention provide novel and advantageousvertically aligned carbon nanotubes (VACNTs) and methods of synthesizingthe same. VACNTs can be synthesized on nickel foam (Ni—F), for exampleby using a plasma-enhanced chemical vapor deposition (PECVD) technique.A wet chemical method can then be used to coat a layer on the VACNTs,for example a layer of nanoparticles such as tin oxide (SnO₂)nanoparticles. SnO₂-coated VACNTs on Ni—F have excellent lithium(Li)-ion storage capacity and can be used as a material for an anode inhigh-performance Li-ion batteries (LIBs).

The SnO₂-coated VACNTs can be denoted as SnO₂-VACNTs, and theSnO₂-VACNTs on Ni—F can be denoted as SnO₂-VACNTs/Ni—F. The VACNTs grownon Ni—F can also be coated or encapsulated with other metals, metaloxides, semiconducting materials, and/or alloys to form compositematerials. The coated or encapsulated VACNTs of embodiments of thesubject invention can have applications in sensors, electron emitters,and energy storage.

High capacity, electrochemically active SnO₂ materials are one of themost promising candidates for replacing graphite in developinghigh-performance LIBs. Despite the tremendous potential of SnO₂ as ananode material, structural pulverization led by the significant volumechanges during electrochemical redox restricts the cycling stability ofLIBs. The conventional bilayer anode design consisting of a binder andconductive additive materials can seriously reduce the usable capacityof LIBs and limit the potential application of LIBs at hightemperatures. Embodiments of the subject invention synthesizebinder-free and conductive additive-free, self-standing SnO₂-VACNTs onthree-dimensional (3D) Ni—F. This material can be directly used, forexample, as anode materials for LIBs without the need for binders,conductive additives, and/or an extra current collector. The coated SnO₂particles can have small diameters (e.g., less than 10 nanometers (nm)or even less than 5 nm), which can shorten the diffusion routes of theLi ions and possibly mitigate the volume change by reducing the strainsduring the Li insertion and extraction. In addition, the free-standingSnO₂-VACNTs with proper spacings can buffer the volume instability andoffer better electrolyte accessibility, thereby providing more favorableLi-ion transportation kinetics at the electrode/electrolyte interfaces.The direct growth of the VACNTs on the Ni—F also provides enhancedmechanical strength, which will improve the stability of theSnO₂-VACNTs/Ni—F when used as LIB anode materials.

Carbon nanotubes (CNTs) have demonstrated the potential for applicationas the anode of LIBs because of their unique one-dimensional tubularstructure, large surface area, short diffusion length of Li⁺ ions, andhigh electrical and thermal conductivity. CNTs are regarded as excellentmaterials to store lithium ions (capacity of about 1000 milliamp-hoursper gram (mAh/g)) compared to the state-of-art graphite anode (372mAh/g). The possibility of using CNTs as the anode material in LIBs forcharge storage has been considered, and CNTs have been excellentadditive materials to improve the electrochemical performance ofelectrode materials with much-improved energy conversion, storagecapacities, and charge transferability.

The experimental specific capacity of CNT anodes has shown to be below400 mAh/g, which is a low value compared to the demands for the highenergy and power densities of LIBs. One of the main reasons causing thelow specific capacity of the CNT anode is its conventional bilayerdesign. A thin layer of CNTs is glued onto a copper foil currentcollector with the help of a binder. The binder limits the potentialapplication of batteries above 200° C., whereas the copper foil itselfis not involved in the electrochemical reaction. The bilayer design ofthe LIB anode can reduce the usable capacity by about 47%, meaning thatLIBs should use free-standing, binder-free CNTs anodes.

High-capacity metal and semiconductor materials such as aluminum (Al)(800 mAh/g), tin (Sn) (994 mAh/g), lithium (Li) (3860 mAh/g), silicon(Si) (4000 mAh/g), and germanium (Ge) (1600 mAh/g) have been explored toimprove the specific and volumetric capacity of LIB anodes. Despitehaving high Li⁺ storage capacity, colossal volume instability during thelithiation/delithiation leads to pulverization of the anode material andloss of inter-particle contact resulting in rapid capacity fading andsevere safety issues. Other anode materials include hybrid materialscomprising high-capacity nanoparticles (less than 100 nm) and CNTs toaddress inherent concerns of bulk electrode materials. The reducedparticle size may modify the volumetric alteration mechanism, andsimultaneously using CNTs can absorb the considerable stress developedduring the lithiation and delithiation process. The high electricalconductivity of CNTs can enhance the performance of LIBs because itoffers a quick pathway for charged particles during the batteryoperation. CNTs coated with SnO₂ nanoparticles (SnO₂-CNTs) as an anodeof LIBs is attractive because the theoretical charge storage capacity ofSnO₂ (781 mAh/g) is over twice as much as that of graphite anodes.

Fabrication of SnO₂-CNTs composites as anode materials focuses on usingwet chemical methods on randomly oriented CNTs. However, SnO₂-VACNTs ofembodiments of the subject invention can surprisingly and significantlyimprove the battery performance by offering better electrolyteaccessibility and charge transfer capability obtained from the orderedstructure and inter-tube space of the VACNTs array. Improved contactresistance between VACNTs and current collectors achieved through thedirect synthesis of VACNTs on metal substrates and anisotropicconductivities of VACNTs can also enhance the charge transfer andadequate dissipation of heat caused by resistive heating. A properinter-tube distance in an array of VACNTs can alleviate the stressdeveloped during the lithiation/delithiation process. Hence, it isessential to fabricate binder-free VACNT templates directly on metalsubstrates to accommodate a uniform coating of SnO₂ nanoparticles forachieving high-performance LIB anodes.

Embodiments of the subject invention provide arrays of free-standing,binder-free VACNTs synthesized directly on 3D nickel foil or Ni—F usinga PECVD method. The as-synthesized VACNTs can be coated (e.g., withnanoparticles such as SnO₂ nanoparticles) to form core-shell structures(e.g., SnO₂-VACNTs) via a wet chemical method. The electrochemicallithiation performance of the pristine VACNTs and SnO₂-VACNTs when usedas an anode material has been evaluated in a half cell configuration(see the examples). The SnO₂-VACNTs composite anode exhibited asignificantly higher specific capacity of about 1891 mAh/g at a currentdensity of 0.1 Amps per gram (A/g) than that of the pristine VACNTs(about 520 mAh/g). Further, the SnO₂-VACNTs composite demonstrated acapacity of more than 900 mAh/g even at a high current density of 1 A/g,which is a significant improvement over the current graphite anodes ofLIBs.

The SnO₂-VACNTs/Ni—F of embodiments of the subject invention haveexcellent Li-ion storage capacity. The material can be used, forexample, as anode materials for developing high-performance LIBs. TheSnO₂-VACNTs synthesized on other metal foam or networks can also haveexcellent Li-ion storage capacity and can be used as anode materials fordeveloping high-performance LIBs. The SnO₂-VACNTs synthesized on Ni—F orother metal foam or networks would also be expected to have excellentsodium-ion storage capacity and can therefore be used as anode materialsfor developing high-performance sodium-ion batteries. The SnO₂-VACNTssynthesized on Ni—F or other metal foam or networks can haveapplications in developing high-performance supercapacitors, and/orsensors (e.g., gas and/or chemical sensors).

By controlling the synthesis conditions in embodiments of the subjectinvention, the density and/or length of the VACNTs can be controlled.Also, by controlling the synthesis conditions, the coating amount, layerthickness, and/or particle size of the SnO₂ on the VACNTs can becontrolled. The techniques can be applied to other metal foam or metalnetworks or metal alloy networks besides Ni—F (for example, iron foam,stainless steel foam, stainless steel mesh, cobalt foam, copper foam,nickel-iron alloy mesh, etc.) to synthesize VACNTs thereon.

Other metals, metal oxides, semiconducting materials, and/or alloys(e.g., ZnO₂, V₂O₅, MnO₂, NiO, Sn, Si, Cu, Ge, NiCo, etc.) can be coatedon the VACNTs by using either a wet chemical method, physical (thermal)vapor deposition, or chemical vapor deposition to form compositematerials. These composite materials can be used for LIBs,supercapacitors, catalysis, sensors, electron emitters, etc.

Embodiments of the subject invention allow for the large-scale synthesisof SnO₂-VACNTs on 3D Ni—F to meet various application requirements. Thecomposite materials of embodiments of the subject invention havepotential applications in bio/chemical sensors, electron emitters,environmental protection (e.g., applications for removing heavy metalions and contaminants from water and purifying the air), energy storage(e.g., applications for LIBs, supercapacitors, solar cells, etc.),energy production (e.g., applications for water splitting to generateoxygen and hydrogen, clean energy sources), and/or synthesis of metal-,metal oxide-, metal sulfide-, alloy-, and/or semiconductor-coated VACNTson Ni—F to form new functional materials.

The following also discuss VACNTs and are each hereby incorporated byreference herein in their respective entireties: U.S. Pat. Nos.10,336,618, 10,968,103, 10,961,123, U.S. patent application Ser. No.17/363,395, U.S. patent application Ser. No. 16/850,687, U.S. patentapplication Ser. No. 17/201,464, Thapa et al. (Direct growth ofvertically aligned carbon nanotubes on stainless steel by plasmaenhanced chemical vapor deposition, Diamond & Related Materials, 90(2018), 144-153), and Jungjohann et al. (Improving field emissionproperties of vertically aligned carbon nanotube arrays through astructure modification, J. Mater. Sci., 55:2101-2117, 2019).

When the term “about” is used herein, in conjunction with a numericalvalue, it is understood that the value can be in a range of 95% of thevalue to 105% of the value, i.e. the value can be +/−5% of the statedvalue. For example, “about 1 kg” means from 0.95 kg to 1.05 kg.

The transitional term “comprising,” “comprises,” or “comprise” isinclusive or open-ended and does not exclude additional, unrecitedelements or method steps. By contrast, the transitional phrase“consisting of” excludes any element, step, or ingredient not specifiedin the claim. The phrases “consisting” or “consists essentially of”indicate that the claim encompasses embodiments containing the specifiedmaterials or steps and those that do not materially affect the basic andnovel characteristic(s) of the claim. Use of the term “comprising”contemplates other embodiments that “consist” or “consisting essentiallyof” the recited component(s).

A greater understanding of the embodiments of the subject invention andof their many advantages may be had from the following examples, givenby way of illustration. The following examples are illustrative of someof the methods, applications, embodiments, and variants of the presentinvention. They are, of course, not to be considered as limiting theinvention. Numerous changes and modifications can be made with respectto the invention.

Materials and Methods

Free-standing, binder-free VACNTs were synthesized on 3D Ni—F via PECVD.An apparatus as shown in FIG. 10 was used, and a detailed description ofthe system can be found in Thapa et al. (2018, supra.) and U.S. Pat. No.10,961,123, both of which are hereby incorporated herein by reference intheir entireties. In brief, as-received Ni—F (1.6 millimeters (mm)thick) was cut into circular disks of diameter about 1 centimeter (cm).They were loaded into the PECVD chamber after being cleanedultrasonically in an acetone and isopropyl alcohol bath for 10 minutes(min), and then the chamber was pumped down to the pressure of 0.01Torr. VACNTs were synthesized at 600° C. for 7 min by using acetylene(25 standard cubic centimeters per minute (sccm)) as a carbon precursorgas in the presence of ammonia gas (400 sccm) while DC plasma of 70Watts (W) was applied at the pressure of 7 Torr. The SnO₂-coated VACNTs(SnO₂-VACNTs) composite was prepared at room temperature by a simplewet-chemical method (see also; Han et al., Coating Single-Walled CarbonNanotubes with Tin Oxide, Nano Letters 3(5) (2003) 681-68; and Li etal., Improving field emission properties of vertically aligned carbonnanotube arrays through a structure modification, Journal of MaterialsScience 55(5) (2020) 2101-2117; both of which are hereby incorporatedherein by reference in their entireties). The as-synthesized VACNTarrays (not separated from original Ni—F) were treated with nitric acid(HNO₃, 20%) for 15 min to create the oxygen-containing functional groupson the VACNT wall. A precursor solution was prepared by mixing 1 gram(g) of tin (II) chloride (SnCl₂, 98%, anhydrous) in 80 milliliters (mL)of deionized water, and then 1.4 mL of hydrochloric acid (HCl, 38%) wasadded. The acid-treated VACNT samples, first rinsed with deionizedwater, were submerged into the solution for 9 hours. The solution wasstirred by the gentle flow of air through the solution continuously tofacilitate the coating process. The samples were taken out from thesolution and dried at 95° C. for 15 min.

The morphology of the pristine VACNTs and SnO₂-VACNTs composite wasanalyzed by using a scanning electron microscope (SEM, JEOL JSM-6330F).Transmission electron microscopy (TEM) images, energy-dispersive X-rayspectra (EDS), and selected area electron diffraction (SAD) patternswere collected by using an FEI Tecnai F30 high-resolution transmissionelectron microscope with an acceleration voltage of 300 kilovolts (kV).The thermal analysis was performed using thethermogravimetric-differential scanning calorimetry analysis (TG/DSC,SDT Q600 V20.9, USA) at a heating rate of 10° C./min under airflow.Structural defects on the VACNTs before and after coating with SnO₂nanoparticles were characterized by Raman spectroscopy (Ar+ laserexcitation, wavelength 632.8 nm). The crystal structure of theSnO₂-VACNTs was confirmed by X-ray diffraction (XRD) experiments usingSiemens Diffraktometer D5000 with Cu (Ka) radiation (λ=1.54 Å) at thestep size of 0.02° ranging from 20° to 80° at a speed of 2°/min. Thefunctional groups created after the acid treatment and SnO₂ coating onthe CNT wall were analyzed using a Fourier transform infraredspectrometer (Jasco, FTIR-4100). CR2032 coin cells (half-cellconfiguration with Li foil as both counter and reference electrode) wereassembled inside an argon (Ar)-filled glove box using pristine VACNTsand SnO₂-VACNTs composites fabricated on Ni foams as the anode. 1M LiPF₆in EC:DEC (1:1, vol. %) was used as an electrolyte; whereas, a Celgardpolypropylene film was used to separate the anode and cathode. Theelectrochemical measurements were carried out on a VMP3 Bio-Logicpotentiostat. FIG. 1 shows the steps of VACNT synthesis, coating withSnO₂, and the Li-ion coin cell fabrication.

EXAMPLE 1

Free-standing, binder-free VACNT templates for accommodating thehigh-capacity SnO₂ nanomaterials were synthesized directly on 3D Ni—F bythe PECVD method. The on-site decomposition of hydrocarbon gas intocarbon atoms, diffusion into the catalyst nanoparticles/nano-hills, andprecipitation on the catalyst surface to enable the CNT graphitizationare essential steps in the VACNT synthesis process. The presence ofcatalytically active growth sites on the substrate is crucial when acatalytic metal substrate is preferred over the substrate coated with athin film of transition metals. FIGS. 2(a)-2(d) show the surfacemorphology of Ni—F before and after the heat treatment at 600° C. in anNH₃ environment for 6 min. As shown in FIGS. 2(a) and 2(c), pristine Nifoam had a relatively smooth surface with an average roughnessR_(a)=53.68 nm. After the heat treatment, the Ni foam was much rougherwith R_(a)=105 nm, as shown in FIGS. 2(b) and 2(d). Most importantly,uniform nano-hills were evolved on the Ni—F surface after the heattreatment under the reducing environment, which was crucial for thenucleation of the VACNTs.

FIG. 3(a) shows an SEM image of a well-aligned, self-standing, anduniform array of VACNTs synthesized on the Ni—F. The inset of FIG. 3(a)depicts a high magnification SEM image of the boxed area shown in FIG.3(a). The diameter of VACNTs ranged from 150 nm to 270 nm, and thelength was about 5 micrometers (μm) for the VACNTs. FIGS. 3(b) and 3(c)show low and a high magnification, respectively, TEM images of VACNTs,which confirmed a tubular “bamboo-like” structure of VACNTs with acatalyst nanoparticle at the tip, a characteristic of the VACNTssynthesized via PECVD at a low temperature. As shown in the inset ofFIG. 3(c), the high-resolution TEM image revealed the lattice fringesseparated by a distance of 0.34 nm in the VACNT wall, indicating themultiwalled CNTs. FIG. 3(d) is a high-resolution TEM image of theinterface between the VACNT wall and the catalyst particle, and it showsthe lattice planes separated by 0.21 nm, which corresponds to the (111)lattice plane of face-centered cubic (FCC) Ni crystal. The typicalelectron diffraction pattern of the Ni nanoparticle trapped at the tipof the VACNT is shown in the upper-right inset of FIG. 3(d). Thedistinct diffraction spots can be assigned to (020), (1 11), and (111)planes of pure FCC Ni along the [101] zone axis. As shown in thelower-left inset of FIG. 3(d), the EDS spectrum confirmed thenanoparticle as a Ni metal nanoparticle, where the carbon (C) and copper(Cu) signals can be ascribed to the VACNT wall and TEM holder,respectively.

FIG. 3(e) shows a uniquely bundled SnO₂-VACNTs structure formed afterthe solution phase SnO₂ coating procedure with VACNT tips touching eachother. The active material (SnO₂-VACNTs) yield was 1.02-1.21 milligramsper square centimeter (mg/cm²) for the VACNTs synthesis period of 6 minand SnO₂ coating duration of 9 hours. Low and high magnification TEMimages shown in FIGS. 3(f) and 3(g), respectively, illustrate thecoating of the VACNT by a thin layer (about 20 nm) of SnO₂nanoparticles, forming a core-shell structure. As shown in FIG. 3(h),the high-resolution TEM image confirmed the crystalline phase of theSnO₂ nanoparticle and demonstrated lattice planes separated by 0.33 nmrelated to the (110) plane of the tetragonal SnO₂. The upper-right insetof FIG. 3(h) represents the typical electron diffraction pattern of theSnO₂-VACNT composite. The concentric diffraction rings can be indexed as(110), (101), and (210) planes related to the polycrystalline phase oftetragonal SnO₂. As shown in the lower-left inset of FIG. 3(h), thepresence of Sn, oxygen (O), and C in the EDS spectrum furtherestablished the material as the SnO₂-VACNT core-shell, where the signalssuch as Ni and Cu can be ascribed to the catalyst particle and TEM grid,respectively.

Thermogravimetric analysis (TGA) was performed to determine the SnO₂loading on the SnO₂-VACNTs composite. FIG. 4(a) shows the TGA profilesof the pristine VACNTs and SnO₂-VACNTs composite sample. Both samplesexhibited typical weight loss due to moisture removal below 400° C.,although the composite showed more significant weight loss, which can beaccredited to the solution-based SnO₂ coating procedure. The compositedisplayed a rapid weight loss starting at about 463° C., and thepristine VACNTs sample showed an abrupt weight loss at about 425° C. Thethermal stability of SnO₂ powder with a particle size of about 10 nm hasbeen shown up to 900° C. The final residual weights of both samples wereexpected to have all the carbonaceous species burned off by 700° C.,leaving behind the dry mass of SnO₂ and any impurities in the VACNTs.The SnO₂ mass loading of the composite was determined to be about 40% ofthe total mass of SnO₂-VACNTs from the TGA spectra, which is the finalresidual mass percentage remaining at 800° C. after subtracting thefinal mass remaining by the VACNTs. As shown by Raman spectra in FIG.4(b), the coating with SnO₂ slightly increased the defects on theVACNTs, which is evident from the increase in the intensity ratio ofD-band and G-band (ID/IG) from 1.12 to 1.18. The higher defects on theSnO₂-VACNTs can be accredited to the treatment of VACNTs with HNO₃ acidbefore being coated with SnO₂. It is important to note that the acidtreatment was crucial to creating oxygen-containing functional groups onthe VACNTs wall to assist the coating process. The crystallographicstructure of the pristine VACNTs and SnO₂-VACNTs composite wascharacterized by XRD, as shown in FIG. 4(c). The XRD spectra show thepeaks (2θ) at about 26.6°, 33.9°, 42.6°, and 51.4°, which can be indexedas (110), (101), (210), and (211) planes, respectively, of thetetragonal phase of SnO₂ nanoparticles. The VACNT related peaks at about26° and 42° further indicated that the test material was the SnO₂-VACNTscomposite.

The functional groups present on the acid-treated VACNTs and SnO₂-VACNTswere examined using FTIR analysis, as shown in FIG. 4(d). Chemicaloxidation with HNO₃ can generate functional groups at the defect sitesof the VACNTs wall. The FTIR spectra of both samples revealed the C═C,C═O, C—N, and N—H stretching vibrations at various wave number positionsbetween 4000 cm⁻¹ and 550 cm⁻¹. Peaks at about 3790 cm⁻¹, 2319 cm⁻¹, and1256 cm⁻¹, associated with N—H and C—N band stretching, can beattributed to adsorbed H₂O and NH₃. The peaks at 2901 cm⁻¹ and 2844 cm⁻¹may be associated with the —CH and C═H band stretching vibrations,respectively. The signals at about 1730 cm⁻¹ and 1633 cm⁻¹ are relatedto carbonyl (C═O) stretching vibration of the carboxylic acid group andcarbon structure (C═C) of VACNTs, respectively. The peaks at 1445 cm⁻¹,1359 cm⁻¹, 1096 cm⁻¹, and 804 cm⁻¹ correspond to the CH₂/CH₃, C—C, C—O,and —OH band stretching, respectively. The signal associated with theSn—O or Sn—OH stretching vibrations at ˜3716 cm⁻¹, 645 cm⁻¹, and 579cm⁻¹ indicated that the carbon nanomaterial contained SnO₂nanoparticles.

EXAMPLE 2

The electrochemical Li-ion storage behavior of pristine VACNTs andSnO₂-VACNTs composite anodes for LIBs was examined using lithium foil asa reference electrode. The electrochemical reactions that occurredduring the cycling process were studied using cyclic voltammetry (CV)tests. FIGS. 5(a) and 5(b) show CV curves of VACNTs and SnO₂-VACNTscomposite anodes measured between 0.01 V and 3 V (versus Li⁺/Li) at ascan rate of 0.2 mV/s for the first five cycles. The electrochemicalreactions of pristine and hybrid anode materials in LIBs can beunderstood as follows. Upon initial charge, the SnO₂ transformed to Snand Li₂O according to the following reaction.

SnO₂+4Li⁺+4e⁻→Sn+2Li₂O   (1)

Subsequent lithiation of the SnO₂-CNT anode corresponds to the followingreversible phase transformation reactions.

Sn+xLi⁺+xe⁻↔Li_(x)Sn(0≤x≤4.4)   (2)

C(nanotube)+xLi⁺+xe⁻↔Li_(x)C   (3)

For both anode materials, sharp irreversible reduction peaks (at about0.96 V for pristine VACNTs and about 0.81 V for SnO₂-VACNTs) during thefirst CV cycle indicated the formation of solid electrolyte interphase(SEI) on the anode surfaces from the decomposition of EC and DEC. Thedistinct reduction peak for the composite anode can also be accreditedto the initial irreversible reduction of SnO₂ to Sn and Li₂O (Equation1). The reversible reduction peak at about 0.27 V for the compositeanode can be associated with the alloying of Li with Sn metal. Besides,reversible oxidation peaks at about 0.39 V for VACNTs can be attributedto the extraction of Li-ions from VACNTs (Equation 3). In contrast,oxidation peaks at about 0.67 V, 1.32 V, and 2.01 V for SnO₂-VACNTs canbe assigned to de-alloying of Li_(x)Sn (Equation 2) and oxidation ofSn⁺². The CV cycles overlapped after the first cycle, indicating thepromising reversibility of electrochemical reactions, which may becrucial for capacity retention and long cyclability of LIBs.

The electrochemical performance of the as-synthesized electrodes wastested by the galvanostatic charge/discharge cycling at a constantcurrent of 0.1 A/g with cut-off potentials at 0.01 V and 3 V versusLi/Li⁺, as shown in FIGS. 5(c) and 5(d). The first charge and dischargecapacities for VACNTs were 535 mAh/g and 1240 mAh/g, respectively.Similarly, the first charge and discharge capacities for SnO₂-VACNTswere 2695 mAh/g and 3927 mAh/g, respectively. However, dischargecapacities for VACNTs and SnO₂-VACNTs in the second cycle were droppedsignificantly to 542 mAh/g and 2581 mAh/g, respectively. In the firstdischarge curves of both electrodes, a plateau is present at about 1.4V, which may be assigned to the irreversible reduction of surfacespecies containing oxygen on VACNTs. Also, the plateau that emerged atabout 0.8 V can be attributed to the formation of an SEI layer inpristine VACNTs, while it can be due to the formation of the SEI andLi₂O in the case of the composite electrode. Hence, the largelyirreversible capacity loss between the first and second discharge cyclesof electrodes was due to the irreversible reactions on the surface ofVACNTs, the formation of the solid electrolyte interphase, and theformation of amorphous Li₂O during the first cycle.

The galvanostatic cycle performance of the pristine VACNTs andSnO₂-VACNTs composite anodes was examined for 100 cycles of charge anddischarge, and the results are shown in FIGS. 6(a) and 6(b). Thecapacity of both anode materials became stable and reversible after theinitial few charge/discharge cycles. With a current density at 0.1 A/g,the LIB with VACNTs anode exhibited an excellent cyclability withcoulombic efficiencies of more than 99% after the first cycle. Thecapacity remained about 520 mAh/g at 0.1 A/g after 100 cycles. Incomparison, the SnO₂-VACNTs anode displayed a considerably high capacitywith an initial discharge capacity of 3927 mAh/g. However, the LIB withthe composite anode suffered a continuous capacity fading, and after 20cycles, and it only preserved a discharge capacity of about 2085 mAh/g.Despite the initial capacity fading, it showed promising cyclabilityafter 20 cycles and showed a discharge capacity of about 1891 mAh/gafter 100 cycles. The initial poor cyclability of the LIB with theSnO₂-VACNTs anode may be attributed to the significant volume change andpulverization of SnO₂ nanoparticles, which led to the anode breakdown.Moreover, both anode materials displayed excellent high-power ratecapability, as shown in FIGS. 6(c) and 6(d). At current densities of 0.2A/g, 0.5 A/g, 1 A/g, 2 A/g, and 5 A/g, the reversible capacities of thepristine VACNTs were about 458 mAh/g, 399 mAh/g, 344 mAh/g, 289 mAh/g,and 256 mAh/g, respectively. The anode retained about 95% of its initialcapacity at 0.1 A/g, as shown in FIG. 6(c). Similarly, the SnO₂-VACNTscomposite had reversible capacities of about 1696 mAh/g, 1463 mAh/g,1294 mAh/g, 1065 mAh/g, and 894 mAh/g at current densities of 0.2 A/g,0.5 A/g, 1 A/g, 2 A/g, and 5 A/g, respectively (FIG. 6(d)). Also, thecomposite anode retained more than 83% of its initial discharge capacityat 0.1 A/g.

Further, the composite anode's cyclability was evaluated at a highcurrent density of 1 A/g for 200 cycles, as shown in FIG. 6(e). Similarto the performance at the low current density, the composite anodedisplayed a continuous capacity fading during the first few cycles.However, after the first ten cycles, the composite anode exhibited ahighly stable cycling performance with a high specific capacity of morethan 900 mAh/g at a high current density of 1 A/g after 200 cycles withan excellent coulombic efficiency, as shown in FIG. 6(e).

Electrochemical impedance spectroscopy (EIS) measurements were performedon the pristine VACNTs and SnO₂-VACNTs composite electrodes using a sinewave of 5 mV amplitude over a frequency range of 100 kilohertz (kHz) to50 megahertz (MHz) to determine the electronic conductivity and Li-iontransportation within electrodes. The Nyquist complex plane impedancemeasurements were carried out before and after running charge/dischargecycles, as shown in FIGS. 7(a) and 7(b). Both Nyquist plots of theVACNTs and SnO₂-VACNTs comprise a semicircle in the high-to-mediumfrequency region and a straight line inclined at ˜45° angle to the realaxis at the low-frequency region. The intercept at the Z′ axis at highfrequency corresponds to the equivalent series resistance (R_(s)), whichrelates to the total resistance of the electrolyte, separator, andelectrical contact. The depressed semicircle in the medium-frequencyrange describes the charge-transfer impedance (R_(ct)) on theelectrode/electrolyte interface. Moreover, the straight line at thelow-frequency region of the Nyquist plot can be attributed to the Lidiffusion process within the electrode.

FIG. 8 shows an equivalent modified Randle's circuit to analyze theimpedance spectra. In the equivalent model circuit, CPE_(ct) and Z_(w)represent the double layer capacitance at intermediate frequencies andthe Warburg impedance associated with Li-ion diffusion, respectively.

FIG. 9 shows a table of parameters after fitting the impedance datausing equivalent Randle's circuit for the VACNTs and the SnO₂-VACNTselectrodes before and after 100 cycles of charge and discharge. Beforecycling, the composite electrode exhibited lower series resistance(R_(s)) and charge-transfer resistance (R_(ct)) than the VACNTselectrode, indicating fast electron transport and fast faradaicreactions at the electrode surface. The depressed semicircle size in themid-frequency range for both electrodes increased after 100 cycles,revealing higher charge transfer resistance after the cycling process.The increase of resistance may be caused by the formation and thickeningof SEI and loss of active materials. In the SnO₂-VACNTs electrode, thecharge-transfer resistance along with series resistance after cyclingincreased significantly than the pristine VACNTs electrode. The increaseof series resistance could be attributed to the formation and extensionof the gap between the SnO₂ particle and CNT wall as a result ofsubstantial volume expansion/contraction and pulverization of the SnO₂during lithiation/delithiation. The continuous pulverization of the SnO₂particle exposes a new grain surface for the fresh SEI formation, whichleads to the thickening of the SEI on the anode material, increasingcharge-transfer resistance. The increase in electrode resistances of thecomposite electrode is well supported by the continuous capacity fadingduring the first few cycles of the SnO₂-VACNTs, as shown in FIGS. 6(b)and 6(e).

Overall, the pristine VACNTs anode exhibited very stable cyclingstability up to 100 cycles with a capacity of about 520 mAh/g at acurrent density of 0.1 Ah/g and excellent rate capability at varioushigh current densities. The SnO₂-VACNTs composite electrode displayed amuch higher capacity of about 1891 mAh/g at a current density of 0.1Ah/g after 100 cycles and a high-rate capacity of about 894 mAh/g evenat a high current density of 5 A/g. The composite electrode also showedlong-term cycling stability for 200 cycles at a high current density of1 A/g with a capacity of more than 900 mAh/g after 200 cycles withexcellent coulombic efficiency, showing potential anode material forhigh-energy and high-power LIBs.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

All patents, patent applications, provisional applications, andpublications referred to or cited herein are incorporated by referencein their entirety, including all figures and tables, to the extent theyare not inconsistent with the explicit teachings of this specification.

What is claimed is:
 1. A method of synthesizing vertically alignedcarbon nanotubes (VACNTs), the method comprising: providing a pristinenickel foam (Ni—F) substrate in a reaction chamber; lowering a pressureof the reaction chamber and providing a carbon precursor gas to thechamber to perform a plasma-enhanced chemical vapor deposition (PECVD)process to synthesize the VACNTs directly on the pristine Ni—Fsubstrate; and performing a physical vapor deposition process or achemical vapor deposition process on the VACNTs connected directly tothe Ni—F substrate to coat the VACNTs with nanoparticles such that theVACNTs have a coating of the nanoparticles on respective outer wallsthereof.
 2. The method according to claim 1, the VACNTs beingmulti-walled VACNTs.
 3. The method according to claim 1, thenanoparticles comprising at least one of tin (Sn), silicon (Si), copper(Cu), germanium (Ge), and nickel cobalt (NiCo).
 4. The method accordingto claim 1, the nanoparticles being tin (Sn) nanoparticles, silicon (Si)nanoparticles, copper (Cu) nanoparticles, germanium (Ge) nanoparticles,or nickel cobalt (NiCo) nanoparticles.
 5. The method according to claim1, the physical vapor deposition or chemical vapor deposition processcomprising providing precursor materials.
 6. The method according toclaim 5, the precursor materials comprising tin (Sn), silicon (Si),copper (Cu), germanium (Ge), and nickel cobalt (NiCo).
 7. The methodaccording to claim 1, the VACNTs being binder-free.
 8. A compoundcomprising: a nickel foam (Ni—F) substrate; vertically aligned carbonnanotubes (VACNTs) disposed on and directly connected to the Ni—Fsubstrate; and a coating of nanoparticles on respective outer walls ofthe VACNTs, the nanoparticles comprising at least one of tin (Sn),silicon (Si), copper (Cu), germanium (Ge), and nickel cobalt (NiCo). 9.The compound according to claim 8, the VACNTs being multi-walled VACNTs.10. The compound according to claim 8, the VACNTs being binder-free. 11.The compound according to claim 8, the nanoparticles being Snnanoparticles, Si nanoparticles, Cu nanoparticles, Ge nanoparticles, orNiCo nanoparticles.
 12. The compound according to claim 8, the VACNTsbeing multi-walled VACNTs, the VACNTs being binder-free, and thenanoparticles being Sn nanoparticles, Si nanoparticles, Cunanoparticles, Ge nanoparticles, or NiCo nanoparticles.
 13. Alithium-ion battery, comprising: a cathode comprising lithium; an anodecomprising the compound according to claim 8; and an electrolytedisposed between the cathode and the anode.
 14. The lithium-ion batteryaccording to claim 13, the VACNTs being multi-walled VACNTs, the VACNTsbeing binder-free.
 15. The lithium-ion battery according to claim 13,the nanoparticles being Sn nanoparticles, Si nanoparticles, Cunanoparticles, Ge nanoparticles, or NiCo nanoparticles.
 16. A method ofsynthesizing vertically aligned carbon nanotubes (VACNTs), the methodcomprising: providing a pristine nickel foam (Ni—F) substrate in areaction chamber; lowering a pressure of the reaction chamber andproviding a carbon precursor gas to the chamber to perform a plasmaenhanced chemical vapor deposition (PECVD) process to synthesize theVACNTs directly on the pristine Ni—F substrate; after performing thePECVD process, performing a physical vapor deposition process or achemical vapor deposition process on the VACNTs directly connected tothe Ni—F substrate to coat the VACNTs with nanoparticles such that theVACNTs have a coating of the nanoparticles on the respective outer wallsthereof, the VACNTs being multi-walled VACNTs, the nanoparticles beingtin (Sn) nanoparticles, silicon (Si) nanoparticles, copper (Cu)nanoparticles, germanium (Ge) nanoparticles, or nickel cobalt (NiCo)nanoparticles, and the VACNTs being binder-free.
 17. The methodaccording to claim 16, the physical vapor deposition or chemical vapordeposition process comprising providing precursor materials, and theprecursor materials comprising tin (Sn), silicon (Si), copper (Cu),germanium (Ge), and nickel cobalt (NiCo).